FERTILIZER SALT INDEX
Virtually all fertilizer materials are salts. When they dissolve in the soil they increase the salt concentration of the soil solution. An increase in salt
concentration increases the osmotic potential of the soil solution. The higher the osmotic potential of a solution, the more difficult it is for seeds or plants to extract soil water they need for normal growth.
Renewed interest in placing fertilizer in or close to the seed row makes it important to remember that an increase in salt concentration in the fertilizer band can cause seed and seedling injury. Placing fertilizer at least two inches away from the seed can usually prevent injury. Excess fertilizer application in a starter band can still produce injury, especially under dry conditions.
Salt index values for several fertilizer materials are listed in Table1. A salt index is calculated by comparing the increase in osmotic potential brought about by addition of that fertilizer material compared to the increase in osmotic potential when an equivalent weight of sodium nitrate is added to water. The salt index of a mixed fertilizer containing N, P and K is the sum of the salt index values (partial salt index) of its components.
The salt index of sodium nitrate is defined as 100. Fertilizer materials with salt indices greater than 100 produce an osmotic potential greater than an equal weight of sodium nitrate. Fertilizers with salt index values less than 100 produce an osmotic potential less than an equal weight of sodium nitrate.
The salt index does not predict the amount of material that will produce injury to crops in a particular soil. It classifies fertilizer material relative to each other and shows which is most likely to cause injury. It is possible to formulate similar grades of mixed fertilizers from different materials that have significantly different salt indices.
Crop tolerances vary widely to increased osmotic potential from fertilizer near the seed. Wheat is moderately tolerant of high-salt conditions while soybeans are
FACT SHEET very sensitive. Corn is intermediate in tolerance. Dry soil conditions as well as fertilizers that produce free ammonia (urea, UAN, DAP) will significantly increase seed and seedling stress leading to injury or possible death. Be aware of the salt index of your starter fertilizer and don't over-stress your young crop.
A & L CANADA LABORATORIES, INC.
Sources: 2136 Jetstream Rd. London, ON N5V 3P5 Salt Index of Fertilizers, 1986, Pm-1274d, Iowa State University Fertilizer Application and Technology, 1999, Meister Publishing Phone: 519-457-2575 Western Fertilizer Handbook, 1985, The Interstate Publishers & Printers Fax: 519-457-2664 [email protected] www.alcanada.com
Fact Sheet No. 141 11/2013 Table 1. Salt Index of Fertilizer Materials and Soil Amendments Material and Analysis Salt Index Partial Salt Index * Nitrogen: Anhydrous ammonia, 82% N 47.1 0.572 Ammonium nitrate, 34% N 104.0 3.059 Ammonium sulfate, 21% N, 24% S 88.3 3.252 Urea, 46% N 74.4 1.618 Urea-ammonium nitrate solution: 28% N (39% ammonium nitrate, 31% urea) 63.0 2.250 32% N (44% ammonium nitrate, 35% urea) 71.1 2.221 Calcium nitrate, 15.5% N 65.0 4.194 Sodium nitrate, 16.5% N 100.0 6.080 Phosphorus: Ordinary superphosphate, 20% P205 7.8 0.390 Triple superphosphate, 45% P205 10.1 0.224 Monoammonium phosphate: 11% N, 52% P205 26.7 0.405 10% N, 50% P205 24.3 0.405 Diammonium phosphate, 18% N, 46% P205 29.2 0.456 Ammonium polyphosphate, 10% N, 34% P205 20.0 0.455 Phosphoric acid, 54% P205 1.613 ** Phosphoric acid, 72% P205 1.754 ** Potassium: Potassium chloride, 60% K20 116.2 1.936 Potassium hydroxide, 83.6% K20 1.015 Potassium nitrate, 13% N, 44% K20 69.5 1.219 Potassium sulfate, 50% K20, 18% S 42.6 0.852 Sulfate of potash-magnesia, 22% K20, 11% Mg, 22% S 43.4 1.971 Monopotassium phosphate, 52.2% P205, 34.6% K20 8.4 0.097 Potassium thiosulfate, 25% K20, 17% S 68.0 2.720 Sulfur: Ammonium thiosulfate, 12% N, 26% S 90.4 7.533 Ammonium polysulfide, 20% N, 40% S 59.2 2.960 Gypsum, 23% Ca, 17% S 8.1 0.247 Magnesium oxide, 60% Mg 1.7 0.002 Magnesium sulfate, 10% Mg, 14% S 44.0 2.687 Miscellaneous: Calcium carbonate, lime, 35% Ca 4.7 0.083 Dolomite, 21.5% Ca, 11.5% Mg 0.8 0.042 Manure salts, 20% 112.7 4.636 Manure salts, 30% 91.9 3.067
* The salt index of a mixed fertilizer containing N, P and K is the sum of the partial salt index per unit (20 lbs) of plant nutrient times the number of units due to each component in the formulation. ** Per 100 lbs of H3PO4
• Fact Sheet No. 141 • 11/2013 A & L Canada Laboratories • 2136 Jetstream Rd. • London, ON N5V 3P5 • 519-457-2575 • www.alcanada.com